Braking during cornering poses special challenges in terms of driving dynamics because different forces (inertia force of the lurching vehicle, centrifugal force of the vehicle on a circular course) have to be accommodated, on the one hand, and the marginal conditions during which braking takes place are variable with time. The underlying problematic nature will be explained in the following with reference to FIGS. 1 and 2.
The embodiment of FIG. 1 shows a situation in which a vehicle, e.g. due to excessive speed, is unable to follow a desired ideal course 107 with a comparatively narrow bend radius because the centrifugal force which urges the vehicle out of the circular course 107 exceeds the sum of the cornering forces of the wheels. This situation in position 102 presents itself to the driver as understeering (because the desired course 107 is followed only insufficiently). Therefore, the driver tends to augment steering (major steering angle of the front wheels) which, however, causes a still more noticeable understeering behavior. This may finally lead to the fact that the vehicle itself oversteers (turns curve-inward) but moves on a remarkably understeered course 109 (positions 103 and 104). It would be desirable in this case that the vehicle follows at least the somewhat less understeering course 108 and would especially not turn inwards so that the position 106 results.
However, previous methods for braking optimization did not permit adjusting such a performance. Practically all modem vehicles are equipped with an ABS (anti-lock system), a wheel-individual control which influences the braking pressure on a wheel brake according to the rolling characteristics of the said wheel. However, this system does not permit producing torques about the vertical axis of the vehicle that act especially favorably because the wheels can only be actuated individually but not in a more comprehensive relationship.
In particular, it can be seen that ABS furthers the understeering behavior. This is because a rolling moment (about the longitudinal axis) of the vehicle is caused by the centrifugal force so that the wheels on the outside of a bend are loaded to a stronger extent and, thus, have a reduced tendency to lock. The result is that they will enter the slip range later so that a greater brake force is produced on the bend-outward wheels and a lower brake force is produced on the bend-inward wheels, and the total result is a yaw torque turning bend-outwards (about the vertical axis) of the vehicle which intensifies the understeering tendency.
An object of the present invention is to provide a method and a device for improving the driving performance of a vehicle when braking during cornering which permit a favorable assistance in braking in conformity with the respective situation.
Especially, the method and the device shall be adapted to the time-variable marginal conditions.
Preferably, implementation of the method and the device shall be simple, more particularly, without increased expenditure in sensors.
FIG. 2 shows a lane change which may be desired, for example, in case it is suddenly necessary to avoid an obstacle (child on the road). Reference numeral 214 designates the desired course, and the vehicle moves alongside the positions 201, 202, 203, and 204. Various situations are passed through consecutively. At the commencement of turning inwards, the situation explained with respect to FIG. 1 may occur (understeering due to excessive speed). Besides, the vehicle may become unstable inasfar as it turns bend-inwards on an understeering course as is shown already in FIG. 1. Thus, optimizing a steering maneuver (smallest possible bend radius to the left) was desirable in the transition from position 201 to position 202, while starting from position 202 the increase of driving stability may be desirable to prevent complete swerving of the vehicle. Because measures for increasing the steerability of a vehicle are not necessarily identical to measures for increasing the stability of the vehicle, it may consequently be desirable that other measures are taken in the transition from 202 to 203 than in the transition from 201 to 202. It is assumed in point 203 that it was possible to avoid an obstacle and steering to an offset track is now desired. A countersteering maneuver will therefore commence. Thus, steering points to a different direction than before. A priori, the same considerations as before exist in a mirror-inverted manner. Nevertheless, it should be taken into account in addition that the dynamics of the second bend (to the right in FIG. 2, transition from 203 to 204) is still influenced dynamically by the `case history`, i.e., the sudden steering to the left (transition from 201 via 202 to 203). An appropriate steering arrangement should take into account all of the above-mentioned observations.
Therefore, the present invention proposes detecting a braked cornering maneuver and structuring it with respect to time. Favorable measures are taken, as the situation may be, according to the time sequence during cornering. These measures are taken especially when instabilities become apparent such as e.g. great amounts of slip on any one or more of the wheels. When unstable braking during cornering lasts longer than a time period which reaches a threshold time period, the braking pressure on any one or more of the wheel brakes is modified with respect to optimizing the vehicle stability.
Further, steerability of the vehicle may be enhanced during unstable cornering maneuvers in the time period before the threshold time period is reached.
The various operating conditions, especially cornering maneuvers and instabilities, can be identified in an embodiment with reference to the wheel sensors and, more particularly, without the use of a steering angle sensor.